Multiple sclerosis (MS) is a neurodegenerative demyelinating, disease of unknown etiology. MS plaques contain both CD4+ and CD8+ T-cells and susceptibility is linked to immune response genes, suggesting that MS occurs when autoreactive T-cells enter the central nervous system (CNS) and attack cells expressing myelin antigens. Myelin basic protein (MBP) is one likely target: for these T-cells because it is an abundant protein in the myelin sheath. Auto&#8209;reactive CD4+ T-cells specific for MBP have been extensively studied in an animal model of MS, experimental allergic encephalomyelitis (EAE). EAE is induced by activating CD4+ T-cells through immunization of animals with MBP. Although this model demonstrates that MBP&#8209;specific T-cells are present in the periphery of healthy animals, the investigators have shown that the repertoire of MBP&#8209;specific T-cells in the periphery is strongly shaped by induction of immune tolerance to MBP in vivo. Some MBP&#8209;specific T-cells are more prevalent in the periphery because they escape tolerance while other MBP&#8209;specific T-cells are tightly regulated by tolerance. The mechanisms that induce tolerance to MBP, and how these mechanisms influence autoimmune disease, are unknown and are the focus of this application. In Aim 1, they will use newly developed TCR transgenic mouse models specific for highly tolerogenic MHC class II&#8209;associated epitopes of MBP to define tolerance mechanisms and their impact on disease susceptibility. In Aims 2 and 3, they will investigate a new area: tolerance and autoimmune potential of MBP&#8209;specific CD8+ cytotoxic T-cells (CTLs). Our recent data show that MBP&#8209;specific CTLs are generated in vivo and regulated by tolerance induction. In this proposal, they show that these MBP&#8209;specific CTLs induce a novel CNS autoimmune disease that differs from EAE and demonstrates new parallels to symptoms of MS. They will investigate mechanisms of tolerance that operate on these CTLs as well as their effector mechanisms and target cells within the CNS.